1. Field of the Invention
The invention in general relates to miniature switches, and more particularly, to a capacitive type MEMS switch useful in radar and other microwave applications.
2. Description of Related Art
A variety of MEMS (microelectromechanical systems) devices are used as switches in radar and communication systems, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance, with a low off capacitance, and a relatively low on impedance with a high on capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint, can operate at high RF voltages and may be constructed by conventional integrated circuit fabrication techniques.
Many of these MEMS switches generally have electrostatic elements, such as opposed electrodes, which are attracted to one another upon application of a DC pull down control voltage. In a capacitive type MEMS switch one electrode is on a movable bridge while the opposed electrode, generally the one with a dielectric layer, is on a substrate member. Upon application of the DC pull down control voltage, the bridge is deflected down and, by the particular high capacitive coupling established, the electrical impedance is significantly reduced between first and second spaced apart RF conductors on the substrate member, thus allowing a RF signal to propagate between the first and second conductors.
With this arrangement, the full pull down voltage appears across the dielectric layer resulting in a relatively high electric field across the dielectric. Over time, this high field may lead to charge accumulation on the surface, as well as in the bulk dielectric. Once the dielectric accumulates enough charge, the switch may fail because the charge causes the switch to remain closed even after the pull down voltage is removed.
Additionally, any presence of water vapor molecules may result in positive ions being formed, due to the electrostatic fields generated, with these positive ions migrating across the substrate and on the dielectric. These positive ions induce corresponding negative charges on the undersurface of the movable bridge and its electrode. Further consequences of these charges include, a pull down voltage shift with time, an incomplete, non uniform pull down across the electrode, resulting in a decrease or increase in capacitance and electrode drop out.
It is a primary object of the present invention to obviate the drawbacks associated with the typical prior art MEMS device.